Why does the moon not revolve and rotate at same time like earth does as it goes around the sun?

Answer 1

Because there is no gravity on the moon so it makes it slower to get back to the Earth. On the Earth since there is gravity it takes us a shorter amount of time to launch into space.

posted by, Adam Lambert

Answer 2

Ah. it sounds as if you are becoming aware of the sidereal/synodic mystery of orbiting things! In fact, for each full cycle of lunar phases, the moon orbits a little more than 360 degrees. It seems impossible; don't those extra degrees each month accumulate somehow and... cause something?

Ignore the moon for a few moments, and just imagine the earth in its orbit around the sun. You are viewing the orbit from space, high in the north, with the sun in the center. The earth is going anti-clockwise in its orbit, and also anti-clockwise in its spin. In your mind, put the earth in the 4 o'clock position. See the spot on your earth where the sun would be exactly overhead (at zenith). Watch the earth turn through a day, and at the same time move the earth to about 1 or 2 o'clock. (We're not working to scale here) Do you see that because the earth is orbiting as well as spinning that the earth has to turn MORE than 360 degrees for that same spot to align itself again with the sun directly overhead? If it's hard to see this, use objects or pencil and paper. If you don't get it immediately, you will as you think about it.

When the earth and other bodies align themselves to the same position they were in the day before relative to the sun (as above) that is the body's synodic period. But you saw that in earth's case re-aligning with the sun happens a little after it turns a full 360 degrees, from your vantage point in space. The full 360 degree revolution from your space perspective, the perspective of the distant stars, is called the sidereal period. So the sidereal day is a little shorter than earth's standard day.

The same thing happens for the moon. The lunar cycles are synodic; they relate to the moon being aligned with earth relative to the sun. This is the 29+ day cycle of lunar phases. The 27+ day period is the moon's sidereal period, the time it takes to orbit exactly 360 degrees. Of course, the difference between the lengths of these two periods is the time the moon orbits after it completes 360 degrees of orbit.

Answer 3

It does! But it keeps one face aimed towards the Earth as it does it. It goes around its axis once every 28 or so days, but it also goes around the Earth in that same amount of time, you see. It is called "tidally locked" as the Earth's gravity is keeping the more massive "half" of the Moon towards the Earth. And of course, as it travels around the Earth as the Earth travels around the Sun, its year is the same as ours. So its "day" is only about 1/13th of a year, instead of 365 like us. In other words, one Lunar day and night is about 28 or so Earth days long, as I said. Having one face of the Moon face the Sun for about two weeks makes it pretty hot after a while, which is why the Apollo astronauts had to go first thing in the Lunar morning before it heated up!

To simplify: it does, but it doesn't look as though it does, from the viewpoint of Earth. This is because it stays aimed at the Earth.

Answer 4

The difference is that, while the Moon revolves around Earth, the Earth and the Moon move around the Sun. The Sun is no longer in the same direction in the sky than a month ago, therefore, from one full moon to the next (i.e., for the Moon to show the Sun its same side) requires more than one full revolution.

Answer 5

A full cycle takes longer than a complete revolution around the Earth. This is because after the Moon does a complete rotation around the Earth, the Sun is no longer in the same direction, and the Moon has to do some "catching up".

Answer 6

One side is always dark; the amount of the dark part we see is dependent upon the angle at which we see it. This angle changes as the moon revolves.